3.3　Chain Structure

As the term implies， macromolecular chemistry deals with compounds whose characteristic properties depend mainly on the extraordinarily large size of the molecules： the molecular weight of most natural and synthetic macromolecular compounds lies between 104 and 107. The compounds are glassy if they are amorphous； they have a fiber structure， especially when they are crystalline； they are rubber-elastic if they have the ability to return spontaneously to the amorphous state after release of the tension which keeps them in a crystalline， or at least oriented state.The state of aggregation（glassy， fibrous， rubber-elastic） of a macromolecular compound depends on the structure of the molecules and on the temperature. The macromolecules themselves can be compared to long thin threads or strings of pearls. They form more or less dense， tangled coils（random coils） which can imbibe solvent， first forming a gel and finally forming a highly viscous solution whose viscosity increases with increasing molecular weight. The simplest form that a macromolecule can have is an unbranched chain. One speaks of a“chain-like”macromolecule or a “chain” molecule， because macromolecules consist of a large number of links， i.e.， they have a chain structure. However， in a real chain the links are not held together in an identical manner from link to link. Furthermore the links can move within each other. It is therefore better to think of a pearl necklace in which the stiffness of the chain is determined by the type of pearls and the material used to string the pearls together. This does not exclude the fact that linear macromolecules in solution have a coiled-up structure similar to a randomly tangled ball of yarn or wire. Because of thermal motion these coils are constantly changing their form. The speed of these changes depends on the temperature. To what extent a macromolecule is stretched or coiled depends on the nature of the chain elements， the type of chain bonding， and the solvent. In the simplest case， the chain-like macromolecule consists of atoms of a single element， as， for example， with polymeric sulfur. Equally simple is a linear polymethylene chain which is obtained through the decomposition of diazomethane or through the polymerization of ethylene. The —CH2—group（—CH2—CH2—in polyethylene） is called the structural element， the repeating unit， or the monomer unit of the macromolecule. The number of structural elements（including the one at the beginning and the end of the chain of the molecule） is known as the degree of polymerization（P）.